Spark plugs are mounted in a combustion system such as an internal combustion engine (an engine) for use in igniting the air-fuel mixture. In general, a spark plug includes an insulator having an axial hole, a central electrode which is inserted in the axial hole, a shell which is provided around an outer circumference of the insulator, and a ground electrode which is provided at a front end portion of the shell to form a spark discharge gap between the center electrode.
Incidentally, in the spark plug, the size of the spark discharge gap is enlarged as the electrodes wear while in use. When the size of the spark discharge gap is enlarged, a discharge voltage required to generate a spark in the spark discharge gap is increased. When the discharge voltage is increased in this way, there are fears that an electric current flows from the center electrode to the shell along the surface of the insulator (a so-called flashover occurs) or a spark (a so-called side spark as one form of flashover) is generated between a front end portion of the insulator and a front end portion of the shell, without generating a normal spark discharge in the spark discharge gap.
In order to prevent the flashover which is a discharge that occurs in any other positions than in the spark discharge gap (an abnormal discharge), it is considered to extend a distance of a path along the surface of the insulator (a creeping distance) among paths extending from the center electrode to reach the shell. As approaches for extending the creeping distance, there are proposed techniques in which a longer leg portion is formed, the outside diameter of the front end portion of the insulator is made relatively large, annular grooves are formed on the surface of the leg portion (for example, refer to Patent Literature 1), and a step is formed on an outer circumferential surface of the leg portion (for example, refer to Patent Literature 2).